Bizare mean values in dark masters

[PieterVuylsteke]

Hi,

I have been making darks for different exposures : 5s,15s,30s,60s,90s,120s,150s and 240s.

I do have a ZWO 2600mm camera, and use gain 0 and offset 10.
That offset 10 generates an offset of 100 ADU in each picture.

Thus, a dark of 30s has mean ADU of 101.4


When I use pixinsight to look at the mean values inside of these darks I do find very bizare values.
For 5s I have a mean adu value of 2480, for 30s 965, for 90s 222 and for 150s 204.

I do not understand that if my darks of 30s have a mean ADU of 101.4 (all 20 of them +/- 0.somethong), how can the mean adu of the dark master then be 965.363 ? Using bias and de-bias do not really change these values.

Looking at a science image with exposure of 30s, uncalibrated, gives a mean ADU of 143.6 ADU, the calibrated one becomes 5144 ADU, with then also quite a lot of stars saturated.

Have been looking in the paper of AstroImageJ, but I do not really find an explanation about the mecanisms of this all.

I would think that a "mean" dark master would have the mean value of the 20 dark photo's ??

Can you please clarify this a bit ?

Kind regards,
Pieter.

[karenacollins]

Hi Pieter,

You didn't mention bias images, so I assume you are not using biases (which is ok if you do not need to scale your darks according to exposure time). In that case, the following is what you should see. The debias option doesn't apply as long as you are not using a master bias.

The master dark should have a mean or median (depending on the option you have selected) value of the raw darks, as you mentioned.

If you are not making progress, please post a screen grab of your DP panel settings.

Karen